The present invention relates to an electronic circuit fabricated as an analogue integrated circuit, and a filter device using the same. More specifically, the invention relates to an electronic circuit such as a frequency-dependent negative resistance (FDNR) circuit and a filter device using the same.
In a case where a filter is fabricated on an integrated circuit, it is possible to realize a filter circuit having no inductor by using an active element. As methods for realizing this, there have been proposed various method, such as a method for using a feedback circuit based on an operational amplifier and a method for using a Gm amplifier.
As one of the design procedures of an active filter, there is a method for carrying out the equivalent conversion using capacitors, active elements and resistors from a ladder-type filter circuit comprising inductance, capacitors and resistors. For example, as shown in FIG. 1, there is a method for carrying out the equivalent conversion into a frequency-dependent negative resistor (FDNR) R called Ford-Girling circuit, and a capacitor C (see IEEE PROC. Vol.128, Pt.G. No. 4, pp.195-197).
FIG. 1 is a circuit diagram illustrating an FDNR conversion. In FIG. 1, the reference number 3 denotes a connecting point at which capacitors C1 and C2 are connected, the reference number 11 denotes a first capacitor (C1), the reference number 12 denotes a second capacitor (C2) and the reference number 13 denotes a first resistor (R0). On the opposite side of the connecting point 3 of the capacitors C1 and C2, the first resistor 13 and an operational amplifier 14 are connected to each other in parallel. The operational amplifier 14 has a positive input, which is connected to an earth terminal 15, a negative input, to which an inverted input 16 is supplied, and an output 17.
FIG. 2 illustrates an example of a tertiary low-pass filter realized using this equivalent conversion. In FIG. 2, resistors 25 and 26 obtained by the equivalent conversion are connected in series between an input terminal 1 and an output terminal 2, and a capacitor 29 obtained by the equivalent conversion is connected in parallel between the resistor 26 and the output terminal 2. Between the resistors 25 and 26, an FDNR circuit 10 is connected in parallel. This FDNR circuit 10 comprises first and second capacitors 11 and 12, each of which is connected in parallel between the input terminal 1 and the output terminal 2, a first resistor 13 provided between one ends of the respective capacitors 11 and 12, and an operational amplifier 14 connected to the first resistor 13 in parallel. The operational amplifier 14 has a positive input connected to an earth terminal 15, an inverted input 16 connected to one ends of trip first capacitor 11 and the first resistor 13, and an operation output 17 connected to one end of the second capacitor 12 and the other end of the first resistor 13. In an LCR ladder-type filter as shown in FIG. 3A, this is converted into a constitution which does not have L as shown in FIG. 3B, i.e. a form obtained by dividing the original ladder-type filter by s. As a result, the inductance is converted into a resistor, the resistor is converted into a capacitor, and the capacitor is converted into an FDNR (a term having s-2 if it is an impedance).
Although the FDNR itself does not exist in a passive element, it is possible to equivalently realize an FDNR using an active element as shown in FIG. 1. The feature of this filter circuit is that a DC path between the input and output of an operational amplifier, which forms the input and output paths and the FDNR, is completely interrupted. If a filter is formed by this FDNR, a DC offset voltage produced in the operational amplifier does not appear at the filter output, so that it is possible to design the filter independently of the DC offset produced in the operational amplifier when the filter is designed. Since the DC offset serves as a false input to deteriorate the S-N ratio of the filter, this is an extremely preferred characteristic for a low-pass filter. In addition, it is possible to independently deal with the DC voltage of the operational amplifier and the DC potential in the signal path, so that the degree of freedom in the circuit design is greatly increased.
Although the aforementioned circuit has many advantageous effects as set forth above, there are the following problems. Although the gain itself in the passing band of this filter is 0 dB, there is a signal gain, for example, of several dD to tens dB, in the output of the operational amplifier forming the FDNR. For that reason, in a case where a relatively large level of signal is processed by a low supply voltage, there is disadvantage in that the signal level at the output of the operational amplifier exceeds the working output range of the operational amplifier, so that the signal is distorted
On the other hand, in a case where this filter is used in a low frequency band such as a base band of a radio transmitter-receiver (from tens to hundreds kHz), the element value forming the filter is increased, and in particular, there is a disadvantage when considering the fabrication of integrated circuits.
That is, the condition of equivalent conversion of the FDNR value D can be expressed by the following formula. EQU D=C1.multidot.C2.multidot.R0 (1)
In a case where an integrated circuit is fabricated, the capacity values C1 and C2 used after the equivalent conversion must be capacity values which can be realized on the integrated circuit, and they can not be so increased for certain reasons of the occupied areas. If the value D is constant, the value R0 is derived by dividing the value D by the values C1 and C2 (these values are also realizable values: about tens pF), and the derived value R0 is a high resistance which is greater than hundreds k.OMEGA.. If this resistor is realized on an integrated circuit, the occupied area is extremely great. In addition, as shown in FIG. 4, in a resistor fabricated in an integrated circuit, a parasitic capacity of several pF to tens pF is distributed to an IC substrate and so forth. Since this resistor forms a negative feedback loop of the operational amplifier, this parasitic capacity causes a phase delay in the feedback loop, so that a desired negative feedback is not carried out. As a result, there are problems in that the operation for alternating signals is unstable and oscillates according to circumstances.